As a technology to estimate an emotional condition of each speaker in a voice call, a technology has been known such that whether or not a speaker (an opposing speaker) is in a state of anger is determined by using the number of backchannel feedback of the speaker (see Patent Document 1 as an example).
As a technology to detect an emotional condition of a speaker (an opposing speaker) during a voice call, a technology has been known such that whether or not the speaker is in a state of excitement is detected by using intervals of backchannel utterance etc. (see Patent Document 2 as an example).
In addition, as a technology to detect backchannel feedbacks from voice signals, a technology has been known such that an utterance section of a voice signal is compared with backchannel data registered in a backchannel feedback dictionary and a section in the utterance section that matches the backchannel data is detected as a backchannel section (see Patent Document 3 as an example).
Moreover, as a technology to record a conversation between two people by a voice call etc., and to reproduce a recorded data of the conversation (the voice call) after the conversation is ended, a technology has been known such that a reproduction speed is changed in accordance with an speech rate of a speaker (see Patent Document 4 as an example).
Furthermore, it has been known that vowels can be used as a feature amount of a voice of a speaker (see Non-Patent Document 1 as an example).    Patent Document 1: Japanese Laid-open Patent Publication No. 2010-175684    Patent Document 2: Japanese Laid-open Patent Publication No. 2007-286097    Patent Document 3: Japanese Laid-open Patent Publication No. 2013-225003    Patent Document 4: Japanese Laid-open Patent Publication No. 2013-200423    Non Patent Document 1: “Onsei (voice) 1”, [online], [searched on Aug. 29, 2015], the Internet<URL:http://media.sys.wakayama-u.ac.jp/kawahara-lab/LOCAL/diss/diss7/S3_6.htm>